Exhaust gas clarification catalyst carrying article

ABSTRACT

The exhaust gas purifying catalyst-supported member comprises a metal carrier layer and a catalyst layer which includes an exhaust gas purifying catalyst and silicon oxide and is directly formed on a surface of the metal carrier. Silicon oxide is contained in the catalyst layer, and therefore, the adhesion of the catalyst layer to the surface of the metal carrier is excellent, so that the catalyst layer resists peeling from the metal carrier even when subjected to oscillations, such as when the exhaust gas purifying catalyst-supported member is used as a catalyst for purifying an exhaust gas from, for example, an internal combustion engine.

TECHINICAL FIELD

The present invention relates to a catalyst-supported member in which acatalyst for purifying an exhaust gas exhausted from an internalcombustion engine is supported. More particularly, the invention relatesto an exhaust gas purifying catalyst-supported member in which acatalyst layer hardly peels off from a surface of a metal substratecarrier.

BACKGROUND ART

In an exhaust gas exhausted from internal combustion engines ofautomobiles or the like, carbon monoxide, incomplete combustionhydrocarbon, nitrogen oxide, etc. are contained, and from the viewpointof environmental protection, decrease of quantities of these substancesis desired. To purify such an exhaust gas, a method of bringing theexhaust gas into contact with a catalyst to reduce the harmful gas isknown. As the catalyst, a noble metal, such as platinum, palladium orrhodium, is effective, and a catalyst-supported member for the exhaustgas wherein such a noble metal catalyst is laminated onto a surface of acarrier such as a stainless steel carrier has been employed.

The catalyst-supported member is loaded on an exhaust pipe from aninternal combustion engine of an automobile or the like. In thecatalyst-supported member, therefore, there is a problem that a catalystlayer laminated on the carrier surface is liable to peel off becauseoscillations are given to the loaded catalyst-supported member wheneverthe automobile is driven. That is to say, the catalyst layer is liableto peel off from the carrier in a short period of time, and the effectof the catalyst-supported member is liable to be lost.

For example, claim 1 of National Publication of International Patent No.524018/2001 discloses an invention of “an article comprising a metalsubstrate having a substrate surface comprising at least one metal oxideselected from the group consisting of alumina and rare earth metaloxides; a catalyst comprising at least one catalyst layer having anouter catalyst layer surface, the catalyst layer supported on thesubstrate surface; the catalyst comprising at least one catalyticallyactive particulate material, wherein the catalyst layer comprises atleast two strata and the outer catalyst layer surface comprisesagglomerates of the catalytically active particulate material”. Also inthe article (metal plate having catalytic action) disclosed in thispublication, however, a layer composed of alumina or an oxide of a rareearth metal is formed on the surface of the metal substrate, and on thislayer, two catalyst layers are further formed. Also in the metal platehaving catalytic action, there is a problem that the catalyst layerpeels off when oscillations are continuously given to the catalystlayer.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a catalyst-supportedmember for purifying an exhaust gas from an internal combustion engineof an automobile having a diesel engine or a gasoline engine.

It is another object of the present invention to provide an exhaust gaspurifying catalyst-supported member, which is a catalyst-supportedmember for purifying an exhaust gas from an internal combustion engineof an automobile or the like, hardly suffers peeling of the catalystlayer and hardly loses a catalytic effect.

The exhaust gas purifying catalyst-supported member of the presentinvention comprises a metal carrier and a catalyst layer directly formedon a surface of the metal carrier, said catalyst layer comprising anexhaust gas purifying catalyst and silicon oxide.

That is to say, in the exhaust gas purifying catalyst-supported memberof the invention, silicon oxide is contained in the catalyst layer. Thesilicon oxide cannot become an exhaust gas purifying catalyst directly,but by introducing it into the catalyst layer, a binder action occurs.In the present invention, therefore, by introducing silicon oxide intothe catalyst layer, adhesion between the catalyst layer and the metalcarrier is improved to make it possible to arrange the catalyst layerdirectly on the metal carrier.

The silicon oxide does not exert a direct catalytic action on an exhaustgas as described above, but even if the silicon oxide is introduced intothe catalyst layer to such an extent that the binder action occurs, thecatalytic activity of the catalyst layer containing the silicon oxide ishardly lowered.

By the introduction of silicon oxide into the catalyst layer, thecatalyst layer can be formed on the surface of the metal carrierdirectly, that is, without interposing a heat-resistant inorganic oxidelayer or the like, and even if the catalyst layer is directly formed onthe surface of the metal carrier, the catalytic activity is not lowered.Moreover, the catalyst layer is stably present on the surface of themetal carrier for a long period of time without peeling of the catalystfrom the metal carrier. Therefore, the exhaust gas purifyingcatalyst-supported member of the invention functions stably for a longperiod of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an example of a section of an exhaustgas purifying catalyst-supported member of the present invention.

FIG. 2 is a sectional view showing an example of an apparatus having amesh filter which is an exhaust gas purifying catalyst-supported memberof the present invention and is to be brought into contact with anexhaust gas from a diesel engine.

FIG. 3 is a sectional view showing an example of a section of aconventional exhaust gas purifying catalyst-supported member.

BEST MODE FOR CARRYING OUT THE INVENTION

The exhaust gas purifying catalyst-supported member of the invention isdescribed in detail hereinafter.

FIG. 1 shows an example of a section of an exhaust gas purifyingcatalyst-supported member of the invention, and FIG. 3 shows an exampleof a section of a conventional exhaust gas purifying catalyst-supportedmember. In these figures, like members are given like numerals as muchas possible.

The exhaust gas purifying catalyst-supported member 10 of the inventionis constituted of a metal carrier 12 and a catalyst layer 16 directlylaminated onto a surface of the metal carrier 12. Between the metalcarrier 12 and the catalyst layer 16 directly laminated onto the metalcarrier surface, such an intermediate layer 14 as seen in theconventional exhaust gas purifying catalyst-supported member 10 is notformed.

As the metal carrier 12 to constitute the exhaust gas purifyingcatalyst-supported member 10 of the invention, a metal that is hardlydamaged thermally and chemically by an exhaust gas from an internalcombustion engine can be employed. Examples of such metals includestainless steel, nickel and titanium. Of these, heat-resistant stainlesssteel is preferable. There is no specific limitation on the shape of themetal carrier 12, and various shapes, such as plate, tube, honeycomb andmesh, are adoptable. In the present invention, it is particularlypreferable to use a heat-resistant stainless steel punched tube or amesh filter. The heat-resistant stainless steel punched tube or the meshfilter has excellent heat resistance, and the heat-resistant punchedtube further shows very excellent exhaust gas purifying performancebecause it has many holes formed by punching and thereby allows thecatalyst to have a large contact area with an exhaust gas. Furthermore,even when the punched tube type catalyst is arranged inside an exhaustpipe from an internal combustion engine, the resistance against theexhaust gas pressure is reduced and the burden to the internalcombustion engine is small, because holes are formed.

The exhaust gas purifying catalyst-supported member of the invention maybe a mesh filter for treating an exhaust gas from a diesel engine or thelike. In FIG. 2, an example of an exhaust gas treating apparatus inwhich a mesh filter for treating an exhaust gas from a diesel engine isincorporated is shown. As shown in FIG. 2, the exhaust gas treatingapparatus 20 is an apparatus to treat an exhaust gas that is exhaustedfrom a diesel engine and flows in the directions of arrows. This exhaustgas treating apparatus 20 comprises a casing 22 having an exhaust gasinlet 21 and an exhaust gas outlet 29 and comprises a rectifying plate23, an oxidation catalyst 24 and a wire mesh filter 25 which arearranged in this order from the upstream of the flow of exhaust gas. Awire for forming the wire mesh filter 25 is the metal carrier of theexhaust gas purifying catalyst-supported member of the invention.

In the exhaust gas purifying catalyst-supported member of the invention,a catalyst layer is directly formed on a surface of the metal carrier.

The catalyst layer is formed from an exhaust gas purifying catalyst andsilicon oxide.

The exhaust gas purifying catalyst comprises a noble metal and activatedalumina. Examples of the noble metals employable for the exhaust gaspurifying catalyst include platinum, palladium and rhodium. These noblemetals can be used singly or in combination. In the present invention,it is preferable to use these noble metals in combination. For example,preferably used are combinations of platinum and rhodium, platinum andpalladium, and palladium and rhodium. In case of, for example, acombination of platinum and rhodium, they are used in a mixing ratio(platinum/rhodium) of usually 20/1 to 1/1 by weight, preferably 10/1 to1/1 by weight. By the use of platinum and rhodium in this mixing ratio,excellent gas purifying effect is exhibited.

In the catalyst layer of the catalyst-supported member of the invention,activated alumina is contained together with the novel metal. Theactivated alumina is a particulate substance having an average particlediameter of usually 0.1 to 200 μm, preferably 5 to 150 μm, and is aporous substance having a specific surface area of usually not less than100 m²/g, preferably not less than 150 m²/g. The aforesaid noble metalis supported on the surface of the particulate activated alumina, and inthis state, they are held on a surface of the metal carrier 12. Thenoble metal supported on the surface of the activated alumina has alarge contact area with an exhaust gas and exhibits a high activity asthe exhaust gas purifying catalyst. The weight ratio between the noblemetal and the activated alumina in the exhaust gas purifying catalyst isin the range of usually 1:1 to 1:35.

In the exhaust gas purifying catalyst-supported member 10 of theinvention, the catalyst layer 16 is directly formed on the surface ofthe metal carrier 12. As for the conventional exhaust gas purifyingcatalyst-supported member 10, adhesion of the catalyst layer 16 to themetal carrier 12 is not good, so that an intermediate layer 14 composedof silicon dioxide or the like is formed between the metal carrier 12and the catalyst layer 16 to improve adhesion between the metal carrier12 and the catalyst layer 16, as shown in FIG. 3. However, theintermediate layer 14 must be formed in a stage different from a stagefor forming the catalyst layer 16, and the production process becomescomplicated. Moreover, even if such an intermediate layer 14 is formed,it is difficult to say that the catalyst layer 16 adheres to the metalcarrier 16 with a satisfactory strength, and when an impact caused bydriving an internal combustion engine is continuously applied to theexhaust gas purifying catalyst-supported member, the catalyst layer 16peels off.

In the present invention, it has been found that if silicon dioxide isadded to the catalyst layer 16, this silicon dioxide becomes anexcellent binder to stably bond the catalyst layer 16 to the metalcarrier 12. The silicon dioxide, however, does not act as a catalyst forpurifying an exhaust gas, so that it is necessary to determine theamount of the silicon dioxide in the catalyst layer 16 in such a rangethat the catalytic action of the exhaust gas purifying catalystcomprising a noble metal and activated alumina is not reduced and theadhesion properties of the catalyst layer 16 to the metal carrier 12 aresufficiently exhibited.

In the exhaust gas purifying catalyst-supported member of the invention,the weight ratio between the exhaust gas purifying catalyst and thesilicon oxide in the exhaust gas purifying catalyst layer is determinedin the range of usually 10:90 to 90:10′, preferably 10:90 to 40:60, morepreferably 20:80 to 40:60, particularly preferably 20:80 to 30:70. Bydetermining the amount of the silicon oxide in such a range, adhesionproperties of the catalyst layer to the metal carrier can be remarkablyenhanced without substantial decrease of the catalytic activity of thecatalyst layer. The amount of the exhaust gas purifying catalyst means atotal amount of the aforesaid noble metal and activated alumina.

The catalyst layer having such composition can be formed by variousprocesses. For example, by a process comprising spraying a solutionhaving the composition of the catalyst layer onto the surface of themetal carrier 12 or a process comprising depositing a catalyst layer onthe surface of the metal carrier 12 by CVD (Chemical Vapor Deposition)or the like, the catalyst layer 16 can be formed directly on the surfaceof the metal carrier 12. Further, the catalyst layer can be also formedby a process comprising dissolving the components for forming thecatalyst layer 16 in a solvent to prepare a solution or finelydispersing them in a solvent to prepare a dispersion, immersing themetal carrier 12 in the solution or the dispersion to deposit thecomponents for forming the catalyst layer 16 on the surface of the metalcarrier 12 and then heating the metal carrier having the thus depositedcatalyst layer-forming components to sinter the catalyst layer-formingcomponents.

The catalyst layer formed by depositing the catalyst layer-formingcomponents from the solution or the dispersion and then sintering themas described above has excellent adhesion to the surface of the metalcarrier, and moreover, because the catalyst layer is made porous bysintering, it has a large specific surface area and thereby exhibitsexcellent catalytic activity. According to the above process, further,it becomes possible to form a catalyst layer of high homogeneity whereinthe catalyst layer-forming components are homogeneously dispersed.

The solution or the dispersion wherein the catalyst layer-formingcomponents are dissolved or dispersed, which is used in the aboveprocess, is for example a nitric acid solution or a hydrochloric acidsolution containing those components. The catalyst layer-formingcomponents can be deposited on the surface of the metal carrier bychanging pH of the solution or the dispersion or heating the solution orthe dispersion to change a state of the solution or the dispersion. Forexample, the metal carrier is immersed in a nitric acid solution whereinthe catalyst layer-forming components are dissolved, and the nitric acidsolution is heated to a temperature of usually room temperature (usually25° C.) to 50° C., preferably 30 to 40° C., to deposit the catalystlayer-forming components on the surface of the metal carrier. Bycontinuously carrying out deposition for usually 1 to 24 hours,preferably 5 to 10 hours, under the above temperature conditions, thecatalyst layer-forming components can be deposited in a desiredthickness.

The metal carrier having the deposited catalyst layer-forming componentsis then calcined. The calcining temperature is in the range of usually300 to 600° C., preferably 300 to 500° C., and the calcining time atthis temperature is in the range of usually 1 to 4 hours, preferably 2to 3 hours. Through the above calcining, a volatile component is removedfrom the catalyst layer, and catalytic activity is imparted to the noblemetal and alumina. Further, the silicon dioxide functions as a binder tounite the metal carrier and the catalyst layer.

The average thickness of the catalyst layer formed as above is in therange of usually 5 to 100 μm, preferably 10 to 40 μm.

The exhaust gas purifying catalyst-supported member of the inventionprepared as above exhibits catalytic activity almost equal to or higherthan that of a conventional exhaust gas purifying catalyst-supportedmember having a catalyst layer that is formed on a metal carrier throughan intermediate layer. As compared with a catalyst layer of the exhaustgas purifying catalyst-supported member that is prepared by theconventional process and has an intermediate layer, the catalyst layerin the invention is extremely strongly bonded to the metal carrier, andwhen the exhaust gas purifying catalyst-supported member of theinvention is irradiated with ultrasonic waves to measure a peel ratio ofthe catalyst layer, the peel area of the catalyst layer is decreased to⅕ to 1/10 the peel area of the catalyst layer of the conventionalexhaust gas purifying catalyst-supported member measured afterirradiation with ultrasonic waves under the same conditions.Furthermore, even when the exhaust gas purifying catalyst-supportedmember of the invention is loaded on an exhaust pipe for an exhaust gasfrom an internal combustion engine, it can be stably used for a longerperiod of time than the conventional exhaust gas purifyingcatalyst-supported member.

INDUSTRIAL APPLICABILITY

In the exhaust gas purifying catalyst-supported member of the invention,a catalyst layer is directly formed on a surface of a metal carrierwithout interposing an intermediate layer. This catalyst layer isextremely strongly bonded to the surface of the metal carrier and hardlypeels off even when oscillations or the like are given. The catalyticactivity of the catalyst layer as the exhaust gas purifying catalyst isalmost equal to or higher than that of the catalyst layer formed on themetal carrier through an intermediate layer.

The exhaust gas purifying catalyst-supported member of the invention hasthe above-mentioned layer structure, and its production process can besimplified.

EXAMPLES

The present invention is further described with reference to thefollowing examples, but it should be construed that the invention is inno way limited to those examples.

Example 1

Holes having a diameter of 2.0 mm were punched into a heat-resistantstainless steel tube (diameter 30 mm, length: 100 mm) having a thicknessof 1 mm at a pitch of 3.5 mm to prepare a punched-tube as a metalcarrier.

The punched tube was immersed in a slurry containing alumina (Al₂O₃) andsilicon dioxide (SiO₂) in a ratio of 30:70 (Al₂O₃:SiO₂), then taken outof the slurry and calcined. Thereafter, the punched tube was immersed ina nitric acid solution containing platinum and rhodium in a ratio of 5:1(platinum:rhodium). The solution was heated to 40° C., and the punchedtube was impregnated with the solution over a period of 16 hours so asto homogeneously distribute platinum and rhodium into thealumina/silicon dioxide.

Then, the punched tube was taken out of the solution and calcined in aheating oven at a temperature of 500° C. for 2 hours to prepare anexhaust gas purifying catalyst-supported member.

In the resulting exhaust gas purifying catalyst-supported member,platinum and rhodium were contained in a ratio of 5:1 by weight in termsof metal, and the metals (platinum+rhodium) and activated alumina werecontained in a ratio of 1:6 by weight in terms of metal.

The ratio between the exhaust gas purifying catalyst and silicon oxidein the catalyst layer of the exhaust gas purifying catalyst-supportedmember was 35:70 by weight.

The amount of the noble metals (platinum+rhodium) in the exhaust gaspurifying catalyst-supported member was 5 g/cm². The specific surfacearea of the activated alumina contained in the catalyst layer was 160m²/g.

Comparative Example 1

An exhaust gas purifying catalyst-supported member was prepared in thesame manner as in Example 1, except that an undercoating layer (maincomponent: silicon dioxide) having a thickness of 30 μm was formed onthe surface of the punched tube and a catalyst layer containing nosilicon dioxide was formed on the undercoating layer.

In the resulting exhaust gas purifying catalyst-supported member,platinum and rhodium were contained in a ratio of 5:1 by weight in termsof metal, and the metals (platinum+rhodium) and activated alumina werecontained in a ratio of 1:6 by weight in terms of metal.

In the catalyst layer of the exhaust gas purifying catalyst-supportedmember, silicon oxide was not contained.

The amount of the noble metals (platinum+rhodium) in the exhaust gaspurifying catalyst-supported member was 5 g/cm². The specific surfacearea of the activated alumina contained in the catalyst layer was 160m²/g.

Evaluation Test

A durability test (20 hours) of the exhaust gas purifyingcatalyst-supported members prepared in Example 1 and Comparative Example1 was carried out using an internal combustion engine exhaust gas of900° C. Then, 50% purification temperatures of CO, HC and NOX weremeasured. As a result, the 50% purification temperatures of CO, HC andNOX in case of the exhaust gas purifying catalyst-supported memberprepared in Example 1 were 280° C., 374° C. and 370° C., respectively,while the 50% purification temperatures of CO, HC and NOX in case of theexhaust gas purifying catalyst-supported member prepared in ComparativeExample 1 were 284° C., 380° C. and 374° C., respectively.

Further, purification ratios of CO, HC and NOX at 400° C. were measuredusing an internal combustion engine exhaust gas. As a result, the 400°C. purification ratios of CO, HC and NOX in case of the exhaust gaspurifying catalyst-supported member prepared in Example 1 were 50.0%,52.0% and 54.5%, respectively, while the 400° C. purification ratios ofCO, HC and NOX in case of the exhaust gas purifying catalyst-supportedmember prepared in Comparative Example 1 were 47.0%, 51.1% and 54.5%,respectively.

As is apparent from the comparison between these values, the catalyticaction and effect of the exhaust gas purifying catalyst-supported memberprepared in Example 1 are nearly equal to those of the exhaust gaspurifying catalyst-supported member prepared in Comparative Example 1.

Then, to the exhaust gas purifying catalyst-supported members preparedin Example 1 and Comparative Example 1, ultrasonic waves (output power:150 W) of 38 kHz were applied over a period of 15 minutes, and weightsof the catalyst layers having peeled were measured.

As a result, the peel ratio by weight in the exhaust gas purifyingcatalyst-supported member prepared in Example 1 was 5.0% by weight,while the peel ratio by weight in the exhaust gas purifyingcatalyst-supported member prepared in Comparative Example 1 reached37.5% by weight.

As is apparent from the results, the quantity of the catalyst layerhaving peeled by the external oscillations in the exhaust gas purifyingcatalyst-supported member prepared in Example 1 was decreased to about1/7 the quantity of the peeled catalyst layer in the conventionalexhaust gas purifying catalyst-supported member having an intermediatelayer.

The results are set forth in Table 1.

Examples 2 and 3

An exhaust gas purifying catalyst-supported member was prepared in thesame manner as in Example, 1, except that the quantity ratio between theexhaust gas purifying catalyst and silicon oxide in the catalyst layerformed on the punched tube was changed to 25:80 (exhaust gas purifyingcatalyst:silicon dioxide (SiO₂), Example 2) or 45:60 (exhaust gaspurifying catalyst:silicon dioxide (SiO₂), Example 3).

The resulting exhaust gas purifying catalyst-supported member wasexamined on the 400° C. purification ratios and the 50% purificationtemperatures in the same manner as in Example 1. Further, to theresulting exhaust gas purifying catalyst-supported member, ultrasonicwaves were applied, and the weight of the catalyst layer having peeledwas determined in the same manner as in Example 1.

The results are set forth in Table 1. TABLE 1 Weight ratio in catalystlayer Exhaust gas Pt/Rh weight Inter- purifying ratio in Amount ofAmount of Al₂O₃ (Pt + Rh)/Al₂O₃ mediate catalyst/Silicon catalyst Pt +Rh in catalyst weight ratio in layer oxide layer supported (g/m²)catalyst layer Ex. 1 none 35/70 5/1 5 30 1/6 Ex. 2 none 25/80 5/1 5 201/4 Ex. 3 none 45/60 5/1 5 40 1/8 Comp. formed  35/70^(*1)) 5/1 5 30 1/6Ex. 1 50% Purification 400° C. Purification Peel Temperature (° C.)ratio (%) ratio CO HC NO_(x) CO HC NO_(x) (wt %) Ex. 1 280 374 370 50.052.0 54.5 5.0 Ex. 2 283 376 369 51.0 51.0 55.3 3.2 Ex. 3 277 374 36553.2 55.0 56.0 6.5 Comp. 284 380 374 47.0 51.1 54.5 37.5 Ex. 1Remarks^(*1))weight of exhaust gas purifying catalyst/weight of intermediatelayer

1. An exhaust gas purifying-catalyst-supported member comprising a metalcarrier and a catalyst layer directly formed on a surface of the metalcarrier, said catalyst layer comprising an exhaust gas purifyingcatalyst and silicon oxide.
 2. The exhaust gas purifyingcatalyst-supported member as claimed in claim 1, wherein the weightratio between the exhaust gas purifying catalyst and silicon oxide inthe catalyst layer is in the range of 10:90 to 90:10.
 3. The exhaust gaspurifying catalyst-supported member as claimed in claim 1, wherein theexhaust gas purifying catalyst in the catalyst layer comprises at leastone noble metal selected from the group consisting of platinum,palladium and rhodium, and activated alumina.
 4. The exhaust gaspurifying catalyst-supported member as claimed in claim 1, wherein themetal carrier is a metal plate selected from the group consisting of astainless steel plate, a stainless steel tube and a stainless steelcorrugated plate.
 5. The exhaust gas purifying catalyst-supported memberas claimed in claim 3, wherein the weight ratio between the noble metaland the activated alumina in the exhaust gas purifying catalyst layer isin the range of 1:1 to 1:35.
 6. The exhaust gas purifyingcatalyst-supported member as claimed in claim 1, which is a mesh filterbrought into contact with an exhaust gas from a diesel engine.